This invention relates to a heat treating apparatus for heat-treating objects to be treated, such as semiconductor wafers, etc.
As a conventional heat treating apparatus of this kind is known a low-pressure CVD (LPCVD) apparatus including a heat treatment furnace exemplified in FIG. 5. As shown in FIG. 5, this LPCVD apparatus includes a heating furnace 20 disposed upright on a base mounted on a frame 10A, a double-wall reaction vessel 30 of quartz housed in the heating furnace coaxially therewith, and a heat-treatment boat 40 to be loaded into and unloaded out of the reaction vessel 30 for mounting a plurality of objects-to-be-heat treated, i.e., wafers, vertically spaced from each other. This heat-treatment boat 40 is coaxially loaded into the reaction vessel 30 to form silicon nitride films or others on the surfaces of the wafers W mounted on the heat-treatment boat 40 at a set reduced pressure.
The reaction vessel 30 includes the double-wall vessel having an outer cylinder 31 having only the lower end opened, and an inner cylinder 32 coaxially inserted in the outer cylinder 31, and a manifold 33 disposed at the lower end of the double-wall vessel. The manifold 33 is made of stainless steel and is tightly engaged with the flange on the flower end of the outer cylinder 31 through an O-ring 34 of a heat-resistant resilient material. The manifold 33 bears the inner cylinder 32 at an extension 33A from the inside thereof and is to engage with a flange of the heat treatment boat 40 through a heat-resistant resilient O-ring 35 to thereby seal the reaction vessel 30 to retain the set reduced pressure.
The manifold 33 further includes an exhaust pipe 33B (of the same material as the manifold 33) connected to an exhaust system, such as a vacuum pump, for evacuating the interior of the reaction vessel 30, a gas feed pipe 33C of a heat-resistant, corrosion-resistant material inserted in the manifold 33 at a position on the exterior thereof offset from the exhaust pipe 33B and bent upward along the inside circumferential wall of the outer cylinder 31 for introducing an inert gas, such as nitrogen gas or others, into the reaction vessel 30, and a source gas feed pipe 33D of a heat-resistant, corrosion-resistant material, such as quartz, bent upward long the inside circumferential wall of the inner cylinder 32 for introducing a heat-treatment source gas (reaction gas). The source gas feed pipe 33D introduces source gases, e.g., dichlorosilane and ammonium for heat-treatment or others. Cooling water passages 33E, 33F are formed circumferentially in the flange at the upper end of the manifold 33 and in a thicker portion at the lower end for the prevention of thermal degradation of the respective O-rings 34, 35 near the cooling water passages 33E, 33F.
The heat treatment boat 40 includes a wafer support 41 having a plurality of notches formed at a set interval vertically therein for supporting a plurality of wafers W, a heat insulator 42 disposed on the lower end of the wafer support 41 for heat-insulating the interior of the reaction vessel 30 on the outside thereof to keep the interior thereof at a set temperature, a magnetic seal shaft 43 connected to the center of the underside of the heat-insulator 42, and a flange 45 having a magnetic seal unit 44 connected to the magnetic seal shaft 43 secured thereto. The wafer support 41 inserted in the reaction vessel 30 is rotated by a magnetic fluid in the magnetic seal unit 44.
The heat-insulator 42 includes four plates 42A of quartz, a support 42B having a plurality of support rods or others which support the plates 42A horizontally at their peripheries at a set vertical interval. The heat-insulator 42 further includes a heat-insulating body 42C which is disposed below the support 42B, filled with quartz wool and evacuated, and a cylinder 42D surrounding the substrates and made of the same material as the substrates 42B, and a base 42E which bears these members.
Next, the formation of a silicon nitride film on a wafer F by this low-pressure CVD apparatus will be explained. First, the interior of the reaction vessel 30 is set at 700.degree.-800.degree. C. by the heating furnace 20, the heat-treatment boat 40 is inserted into the reaction vessel 30, and the reaction vessel 30 is sealed, and then the interior of the reaction vessel 30 is evacuated to a set low pressure by the exhaust pipe 33B. Then ammonium and dichlorosilane as source gases are introduced into the reaction through the source gas introduction pipe 33D to form a silicon nitride film on the wafer F by the reaction the ammonium with dichlorosilane. After the film formation is over, an inert gas, such as nitrogen or others, is introduced into the reaction vessel 30 through the gas introduction pipe 33C to replace the gas in the reaction vessel with the inert gas and purge the reaction vessel therefrom, and the pressure in the reaction vessel 30 is returned to atmospheric pressure, and the heat-treatment boat 40 is unloaded from the reaction vessel 30.
In the conventional low-pressure CVD apparatus of FIG. 5, the heat insulating body 42C has good heat-insulating performance, but the hollow container of the heat insulating body 42C is not perfectly sealed at the evacuation portion. Air leaks there, which results in the problems that the film forming reaction is impaired, the heat-insulating performance is lowered, and cracks are caused in the evacuation portion, which damage the heat insulator 42C. This heat-insulating body 42C has such good heat-insulation that the manifold 33 and the sealing flange can be kept at below 100.degree. C. and prevent the O-rings 34, 35 from heat degradation, but reaction gases between the ammonium and dichlorosilane, etc. in the reaction vessel 30 are apt to have side-reactions to resultantly generate particles, such as ammonium chloride, etc. Yields of the heat-treatment of the wafers are lowered by the particles. Especially in micronized processing in sub-microns, as in 4 MDRAMs, it is a problem that the generated particles adversely affect the film formation.